1. Field of the Invention
The present invention relates to an apparatus and method for reducing mottling in printing presses. In particular, the present invention relates to form rolls which reduce solid mottling in an offset printing press.
2. Description of the Prior Art
Offset printing presses are well known in the printing art. In an offset printing press, an image or plate cylinder which contains on its outer surface an image to be printed rotates in contact with either the web or sheet upon which the image is to be printed, or preferably, rotates in contact with a transfer or blanket cylinder which transfers the image from the image cylinder to the web or sheet. Ink is transferred to the image cylinder, which ink is thereafter transferred either directly to the web or sheet or to the transfer cylinder and then to the web or sheet.
There are several known ways in which ink may be fed to an image cylinder in an offset printing press. The most common way of feeding ink to an image cylinder is through the use of an ink fountain. An ink fountain is a device in which a fountain roll has mounted against it a trough which is filled with ink. At the interface between the bottom of the trough (i.e., the ink fountain) and the fountain roll are located a series of ink keys. The ink keys may be moved toward and away from the fountain roll, and the distance between the end of each ink key and the surface of the fountain roll determines the thickness of ink applied by the ink key on the outer surface of the fountain roll. The fountain roll is in close proximity with, in an area past the ink keys in the direction of rotation of the fountain roll, a metering roll.
The metering roll feeds ink to an ink train of one or more rolls, which may include distribution rolls, vibrating rolls, swing rolls, and a form roll. The form roll transfers ink onto the plate cylinder. Prior art form rolls have been made with a outer elastomeric surface which is relatively soft, i.e., has a hardness on the order of 22-28 Shore A durometer hardness. The outer elastomeric surface is cylindrical and of a uniform outer diameter.
One disadvantageous result which can be found in prior art offset printing presses is solid mottle. Solid mottle is a condition of a printed image in which the ink is not distributed across the paper in a uniform manner, thereby resulting in a mottled image which does not have proper optical density across the entire image. In order to compensate for solid mottling, it has often been necessary in prior art devices to increase the amount of ink transferred to the paper.